1. Field of the Invention
The invention relates to a circuit board unit including a plurality of substrates, and a method of connecting circuit boards to each other, and more particularly to a structure for connecting flexible printed circuit (FPC) boards includes in various electronic devices, or printed circuit boards such as a rigid printed circuit (RPC) to each other, and a method of connecting them to each other.
2. Description of the Related Art
In an electronic mobile terminal such as a mobile phone or PDA (personal digital assistant), a plurality of printed circuit boards on each of which a lot of electronic components are mounted has been conventionally arranged in a limited space. For electrically connecting such circuit boards to one another, there is usually used a connector or bonding medium such as solder.
For instance, Japanese Patent Application Publications No. 4-324264 and 2001-24299 have suggested a connector for connecting a flexible cable to a printed board.
FIG. 9 is a plan view of an example of a conventional circuit board unit including a plurality of circuit boards electrically connected to one another.
As illustrated in FIG. 9, the circuit board unit includes two RPC substrates 1a and 2a electrically connected to each other through two connectors 10a and 20a. Specifically, the two RPC substrates 1a and 2a are electrically connected to each other with being kept in parallel with each other. For instance, a male connector 10a is arranged on the RPC substrate 1a, and a female connector 20a is arranged on the RPC substrate 2a. The RPC substrates 1a and 2a are connected to each other through the connectors 10a and 20a. The connectors 10a and 20a may be designed to include terminals to be mounted on the RPC substrates 1a and 2a, arranged in a matrix.
FIG. 10 is a perspective view of another example of a conventional circuit board unit.
As illustrated in FIG. 10, a connector 31 is mounted on a RPC substrate 1a, and a FPC substrate 30 is connected to the RPC substrate 1a by being inserted into the connector 31.
In both the circuit board units illustrated in FIGS. 9 and 10, since the substrates can be readily separated from each other, a substrate found defective can be exchanged to a non-defective substrate.
FIG. 11 is a perspective view of still another example of a conventional circuit board unit.
As illustrated in FIG. 11, a FPC substrate 30 including a connector 32 is connected directly to a RPC substrate 1a. 
As medium for connecting the substrates to each other, there are usually used solder, an anisotropic conductive film (ACF), or anisotropic conductive paste (ACP).
The above-mentioned solder is identical with solder generally used for mounting an electronic component onto a substrate. A method of connecting substrates to each other through solder includes coating solder paste onto electrode terminals of the substrates, temporarily fixing the substrates to each other, and applying heat and pressure to portions of the substrates at which the substrates make contact with each other, to thereby fix the substrates to each other.
The above-mentioned anisotropic conductive film (ACF) is a film containing a mixture of electrically conductive minute particles and adhesive resin, and the above-mentioned anisotropic conductive paste (ACP) is paste containing the same. Hence, when the substrates are connected to each other, an anisotropic conductive film (ACF) or anisotropic conductive paste (ACP) is sandwiched between the substrates, and then, heat and pressure are applied to ACF or ACP to thereby fixedly connect the substrates to each other, similarly to a process of connecting the substrates to each other through solder.
Furthermore, since solder is used for mounting other components onto a substrate, it would be possible to lower costs.
However, since solder becomes liquidized when used, it is likely to short-circuit adjacent electrodes to each other. Accordingly, solder is not suitable to electrical connection of electrodes arranged in a small pitch. A problem would be caused, if solder were used for electrical connection of electrodes arranged at a pitch of 0.3 mm or smaller.
In contrast, though the above-mentioned anisotropic conductive film (ACF) or anisotropic conductive paste (ACP) costs higher than solder, they make it possible to electrically connect electrodes arranged in a small pitch, to each other. For instance, they may electrically connect electrodes arranged in a pitch of 0.05 mm, to each other. Thus, a glass substrate and a liquid crystal driver in a liquid crystal display device are electrically connected to each other often through an anisotropic conductive film (ACF) or anisotropic conductive paste (ACP).
As mentioned earlier, it is necessary in an electronic device such as a mobile phone or a PDA terminal to mount a plurality of printed circuit boards on each of which a lot of electronic components are mounted, in a limited space. In particular, since a mobile phone and a PDA terminal are often carried in a pocket or a bag, it is quite important to thin and save a space of a connection area at which substrates are connected to each other. In order to thin and save a space of an electronic device, it is beneficial to select connection medium such as solder or an anisotropic conductive film (ACF) among the above-mentioned conventional connection mediums.
Since LSI to which the latest technology is applied is frequently mounted on electronic devices, a rate at which defectiveness is found in components is relatively high. Hence, if solder is used, it would not be possible to reuse substrates connected to each other though solder, if a defectiveness is found in the substrates. This results in an increase in fabrication costs.
If a connection medium which is detachable from a substrate, such as a connector, is selected, it would be possible to remove a substrate on which a defective component is mounted, preventing an increase in fabrication costs, though a height of a device at a location where the connector is mounted is sacrificed.
However, a connector is accompanied with problems with respect to electrical connection, as mentioned below.
First, since a connector accomplishes electrical connection by making contact at a terminal pin thereof with an electrode terminal of a printed circuit board, it would be necessary to prepare a space of a terminal pin of the connector and an area the terminal pin is fixed, resulting in the limitation in narrowing a pitch between adjacent terminal pins. For instance, an upper limitation in a pitch in a FPC connector is 0.3 mm, and the same in a connector for connecting substrates to each other is 0.4 mm.
As a connector for reducing an area necessary for being mounted on a substrate, there may be used a CSP type substrate-to-substrate connector. The connector reduces an area necessary for being mounted on a substrate, by arranging electrodes of a substrate on which the connector is to be mounted, in a matrix.
If electrode terminals were arranged in a matrix, it would be necessary to form a wire extending from the electrode terminals into inner layers of a substrate. Furthermore, if a via-hole is formed at an electrode terminal, the electrode terminal is recessed at a surface thereof by about 15 to 40 micrometers. Thus, if a CSP type connector having high rigidity is mounted on a substrate, there occurs a void, defectiveness in contact, or stress concentration at a via-hole, resulting in reduction in reliability.